Archive for October, 2010

Up until now, whenever I’ve needed to power something up, I’ve had two options: Either build it on a solderless breadboard with a breadboard power supply (shameless plug), or Frankenstein something together with a spare computer PSU. With extra molex connectors and such sprawling all over the kitchen counter workbench.

This will make things a bit cleaner.

Take one suspiciously-cheap power supply, subtract some connectors, and add banana jacks and a little heatshrink, and I have myself a nice fixed-output bench power supply. It’s not much of a hack, but it’ll make life much easier for me.

As for the insides…

The funny thing is this power supply is advertised as a 500W unit with two 12V rails, each rated for 13A. In actuality, it only has a single 12V rail, and the rectifier on that rail is only rated for 12A.

Part of the funding for AllgaierShops comes from a side hobby of mine–fixing non-working LCD monitors. I buy them typically off eBay and sell them locally. Occasionally, I come across a monitor that I simply can’t figure out. Such is the case with a Dell e172FPb. One side of the backlight inverter refuses to power a bulb, and after a second, the controller shuts down on undercurrent. I can’t figure out why it’s not working.

So I’m upgrading it to LED backlighting. Yeah, it’ll cost me more to do the backlighting than I originally spent on the monitor, but it’s a cool project, a technical challenge, and a good chance to practice my surface-mount soldering skills.

Here’s the board I designed and had fabricated:

It’s 3″ long and 0.3″ wide, and holds 12 LEDs (0.25″ spacing). I didn’t know how closely I would need to space them in order to get sufficient and even light distribution, so I went on the generous side. I had previously tried out a flexible LED strip I purchased off eBay, with LEDs spaced every 1/2″ or so, and it was quite visible where the LEDs were (and weren’t). The LEDs are grouped in strings of three, and each board can be configured (by selective placing of components) to have either four strings of 3 LEDs or two strings of 6 LEDs, depending on the supply voltage.

Here it is, populated with LEDs:

The first string of three is tied to positive voltage, and will have a current-limiting resistor (R3) tied to ground. The second string is tied to ground and will have a current-limiting resistor (R2) tied to positive voltage. Alternatively, a single resistor (R4) can be placed to join the two strings and limit current for the combined 6-LED string.

Why only a 3″-long PCB? Because I’m still working with the free version of Eagle, and so I’m quite limited on board space. For most of my projects, this isn’t an issue, but when you need a 12″-long PCB, it’s a problem. So I added a pair of pads on each end of each PCB, to make it easy to daisy-chain PCBs into a single, long string:

I used the straight edge of one PCB to line up each joint. A couple big blobs of solder at each end hold the pieces together nicely. Sure, it isn’t the strongest mechanical bond, but once they’re inside the LCD panel, that won’t be an issue. Here’s the finished product:

And, just so you can see how far my surface-mount soldering skills have to go, here’s a closeup:

One nice thing about these boards is that you can cut one in half, in case your application calls for a dimension which is nowhere near a nice multiple of 3″. Like this one, which requires a 13.5″ bulb. I’ll try it with the current 12″ backlight before deciding whether to pursue it.